The present invention is directed to ink compositions. More specifically, the present invention is directed to aqueous based ink compositions suitable for use in ink jet printing systems. One embodiment of the present invention is directed to an ink composition which comprises an aqueous liquid vehicle, a colorant, and a polymeric additive of the formula ##STR2## wherein R.sup.1 and R.sup.2 are independently selected from the group consisting of hydrogen, alkyl groups with from 1 to about 8 carbon atoms, and alkoxy groups with from 1 to about 8 carbon atoms, R.sup.3 and R.sup.4 are independently selected from the group consisting of alkyl groups with from 1 to about 4 carbon atoms, and x and y are each independently a number of from about 100 to about 400, present in an amount of at least about 1 part per million.
Ink jet printing systems generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging, or deflection, the system is much simpler than the continuous stream type. There are two types of drop-on-demand ink jet systems. One type of drop-on-demand system has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the system's ability to produce high quality copies. Drop-on-demand systems which use piezoelectric devices to expel the droplets also suffer the disadvantage of a slow printing speed.
The other type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets and allows very close spacing of nozzles. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink in the immediate vicinity to evaporate almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. When the hydrodynamic motion of the ink stops, the process is ready to start all over again. With the introduction of a droplet ejection system based upon thermally generated bubbles, commonly referred to as the "bubble jet" system, the drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous stream counterparts, and yet have substantially the same high speed printing capability.
The operating sequence of the bubble jet system begins with a current pulse through the resistive layer in the ink filled channel, the resistive layer being in close proximity to the orifice or nozzle for that channel. Heat is transferred from the resistor to the ink. The ink becomes superheated far above its normal boiling point, and for water based ink, finally reaches the critical temperature for bubble formation or nucleation of around 280.degree. C. Once nucleated, the bubble or water vapor thermally isolates the ink from the heater and no further heat can be applied to the ink. This bubble expands until all the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor, which removes heat due to heat of vaporization. The expansion of the bubble forces a droplet of ink out of the nozzle, and once the excess heat is removed, the bubble collapses on the resistor. At this point, the resistor is no longer being heated because the current pulse has passed and, concurrently with the bubble collapse, the droplet is propelled at a high rate of speed in a direction towards a recording medium. The resistive layer encounters a severe cavitational force by the collapse of the bubble, which tends to erode it. Subsequently, the ink channel refills by capillary action. This entire bubble formation and collapse sequence occurs in about 10 microseconds. The channel can be refired after 100 to 500 microseconds minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to become somewhat dampened. Thermal ink jet processes are well known and are described in, for example, U.S. Pat. No. 4,601,777, U.S. Pat. No. 4,251,824, U.S. Pat. No. 4,410,899, U.S. Pat. No. 4,412,224, and U.S. Pat. No. 4,532,530, the disclosures of each of which are totally incorporated herein by reference.
Known ink jet inks generally comprise a water soluble dye which is soluble in an ink vehicle such as water or a mixture comprising water and a water soluble or water miscible organic solvent. For example, U.S. Pat. No. 4,184,881 (Bradley) discloses an ink composition for use in ink jet printing comprising an aqueous solution of a water soluble dye and a humectant consisting of ethylene oxide adducts of at least one acetylenic diol in the absence of any glycol or glycol ether. In addition, U.S. Pat. No. 4,337,183 (Santiago) discloses an aqueous printing ink composition which comprises a physical mixture of polyurethane resin, polyethylene resin, and water as the vehicle. The invention provides a family of water based inks with enhanced physical and mechanical properties, superior abrasion resistance, and good adhesion to various substrates. Further, U.S. Pat. No. 3,477,862 (Forsyth) discloses an ink comprising a dyestuff, a solution of high molecular weight polyethylene oxide and glycerin for employment in a pen, nozzle or other ink applying means to ensure the inscription of a clear continuous solid line on a chart on which the ink applying means is associated as the ink applying means traverses the chart.
Heterophase ink jet inks are also known. For example, U.S. Pat. No. 4,014,833 (Story) discloses a composition and method for improving the ink transfer properties of aqueous printing inks. The composition is an aqueous ink containing from 0.1 to 1.5 percent by weight of a polyethylene oxide resin having a molecular weight in the range of from 100,000 to 350,000. In addition, U.S. Pat. No. 4,680,332 (Hair et al.) discloses a heterophase ink composition which comprises a water insoluble polymer dispersed in a liquid medium, the polymer containing therein an oil soluble dye, and a nonionic stabilizer permanently attached thereto. The polymer may include styrene, parachlorostyrene, vinyl naphthalene, and acrylates wherein the carbon chain length is from about 1 to about 18 carbon atoms. The stabilizers may include ethylene oxide and propylene oxide block copolymers. Further, U.S. Pat. No. 4,705,567 (Hair et al.) discloses a heterophase ink jet ink composition which comprises water and a dye covalently attached to a component selected from the group consisting of poly(ethylene glycols) and poly(ethylene imines), which component is complexed with a heteropolyanion. In addition, U.S. Pat. No. 4,597,794 (Ohta et al.) discloses an ink jet recording process which comprises forming droplets of an ink and recording on an image receiving material by using the droplets, wherein the ink is prepared by dispersing fine particles of a pigment into an aqueous dispersion medium containing a polymer having both a hydrophilic and a hydrophobic construction portion. The hydrophilic portion constitutes a polymer of monomers having mainly polymerizable vinyl groups into which hydrophilic portions such as carboxylic acid groups, sulfonic acid groups, sulfate groups, and the like are introduced. Pigment particle size may be from several microns to several hundred microns. The ink compositions disclosed may also include additives such as surfactants, salts, resins, and dyes.
Although known compositions and processes are suitable for their intended purposes, a need remains for improved ink compositions. A need also remains for ink compositions suitable for use in ink jet printers. In addition, there is a need for ink compositions that when used in ink jet printing processes result in uniform drop size over varying drop frequencies. Further, a need exists for ink compositions that when used in ink jet printing processes result in uniform drop speed over varying frequencies. Additionally, a need remains for ink compositions that when used in ink jet printing processes result in relatively large drop sizes; large drop sizes are desirable because they enable greater area coverage on the receiver sheet, provide greater efficiency, and require less heat increase, thereby enabling the use of smaller heaters in a thermal ink jet printing apparatus. In addition, a need exists for ink compositions that when used in ink printing processes result in few or no detectable missing drops. Further, a need remains for ink compositions that when used in ink jet printing processes result in reduced drop satellites; drop satellites are secondary drops that issue from the nozzle or channel after the primary drop has been ejected, resulting in undesired spots on the receiver sheet. Additionally, there is a need for ink compositions having the above advantages accompanied by acceptable latency, surface tension, and viscosity values. There is also a need for ink jet printing processes with inks exhibiting the above noted advantages. A need also exists for thermal ink jet printing processes using inks exhibiting the above noted advantages. Further, there is a need for ink compositions that exhibit short drop transit times and enable good drop directionality when used in thermal ink jet printing processes. Additionally, there is a need for ink compositions that result in reduced or eliminated jitter when used in thermal ink jet printing processes; jitter refers to the degree of variation in drop speed, and reduced jitter entails more uniform drop speeds during the printing process.